Thermoplastic copolyimides

ABSTRACT

Aromatic thermoplastic copolyimides made from a 3,3&#39;&#39;,4,4&#39;&#39;benzophenonetetracarboxylic acid derivative and two or more aromatic diamines, at least one of which is meta-substituted and both or all of which are either meta- or para-substituted, together with precursors for and methods of preparing and using such copolyimides and precursors.

iliiiied Siaies Piei 1191 Acie, Jr. Ar. 10, 1973 [54] THE OPLASTHC COPOLT l t" [56] References Cited UNITED STATES PATENTS [75] Inventor: LuisAcle, Jan, San Diego, Calif.

3,347,808 10/1967 Lavin 6t81. [73] Assignee. International HarvesterCompany, 3,349,061 10/1967 pmckmayrm San Dlego Cahf- 3,410,826 11/1968Endrey 22 i July 3 1972 3,506,583 4/1970 Boram et a1.

3,51 1,790 5/1970 De Brunner et a1. ..260/2.5 [21] Appl. No.: 268,859

- Primary Examiner-Wi11iam H. Short R lat d A e e U S pphcanon DamAssistant Examiner-L. L. Lee [63] Continuation-impart of Ser. No.835,226, June 20, At1 rn y B n E, l of tedt 1969, abandoned.

57 ABSTRACT [52] U.S. Ci. ..260/65, 117/161 P, 161/227, 1

252/1883, 260/25 N, 260/ 32.6 N, 260/328 Aromatic thermoplasticcopolyimides made from a N, 260/334 R, 260/37 M, 260/37 N, 260/783,3,4,4-benzophenonetetracarboxylic acid derivative TF and two or morearomatic diamines, at least one of [51] Hm. Cl. ..C08g 20/32 hich is mta-substituted and both or all of which are [58] Field of Search..260/47 CP, 65, 78 TF, either metaor para-substituted, togetherwithprecursors for and methods of preparing and using such copolyimides andprecursors.

16 Claims, No Drawings THERMOPLASTIC COPOLYIMHDES This application is acontinuation-in-part of application Ser. No. 835,226 filed June 20, 1969and now abandoned.

This invention relates to aromatic copolyimides and, more particularly,to aromatic copolyimides which have the useful properties of previouslyknown polyimides, but which are thermoplastic and can accordingly beformed into useful articles by the techniques employed for forming otherthermoplastic materials.

Aromatic polyimides are extremely valuable materials because of theirchemical inertness, strength, resistance to extremely high temperatures,etc. However, aromatic polyimides have not been widely employed becausethose heretofore known cannot be fabricated using thermoplastic formingtechnology. As a result of this characteristic, they can be shaped intouseful articles only by a limited number of techniques, all of which arecomparatively expensive in relation to those by which thermoplasticmaterials can be fabricated.

An attempt has been made to solve this problem of intractability byforming an aromatic homopolyimide via a reaction in which an aromaticdianhydride and an aromatic diamine are reacted at a high temperature inan inert solvent and in the presence of a tertiary amine catalyst. US.Pat. No. 3,422,064 issued Jan. 14, 1969, to W.G. Gall for AromaticPolyimide Preparation.)

This procedure purportedly produces a molding powder which can becoalesced at a temperature below the crystalline melting point of thepolyimide.

However, it is admitted in the Gall patent that coalescense of themolding powder so produced requires pressures of 2,000 to 30,000 psi.Since the presses now commercially available are capable of producingpressures only up to 2,000 psi, such materials have little practicalvalue. Moreover, the procedure by which they are made would make themtoo expensive to be commercially attractive as it involves solventseparation and other steps.

Another attempt to solve the problem of intractability appurtenant toknown polyimides involves the formation of a copolyimide from anaromatic tetracarboxylic acid and two aromatic diamines, one of whichalso has an aliphatic moiety. U. S. Pat. No. 3,424,718 issued Jan. 28,1969, to R. J. Angelo for Copolymers of Aromatic Tetracarboxylic Acidswith at Least Two Organic Diamines.) While such copolymers have thedesired thermoplasticity, they also have markedly inferior thermalstabilities because of the presence of the aliphatic moiety andaccordingly lack one of the very characteristics which make aromaticpolyimides of such great potential value.

A further attempt to solve the problem of intractability U. S. Pat.No..3,4l0,826 issued Nov. 12, 1968, to Andrew L. Endrey for Process forPreparing Shaped Articles from Polyamide Acids Before Converting toInsoluble Polyimides.) involves the formation of a polyamide acidsolution via a reaction in which an aromatic diamine and a dianhydrideare reacted at low temperature in a solvent which will not react withthe dianhydride or the diamine. The polyamide acid is then formed into afilm or filament; and a tertiary amine catalyst and an acid chloride oranhydride converting agent are added to cause gellation of the shapedmaterial. Finally, the polyamide acid is converted to a polyimide byheating the film or filament.

Onedisadvantage of the Endrey process is that the catalytic solventswhich are employed to effect polyimide formation cause discoloration ofthe final polymer. Also, these solvents are relatively expensive, whichdetracts from the commercial attractiveness of the process.

A further disadvantage of the Endrey process is that the precursors haveonly a very short shelf life because the catalytic solvents cause thepolymerization reactions to proceed at such a rate that the precursorrapidly becomes impossible to handle, even if the precursor is kept atambient temperature.

Additionally, the Endrey materials, being in solution, are limited touse in applications such as the formation of foams, films, andfilaments; coatings; and impregnation. Finally, Endrey is not concernedwith the making of thermoplastic copolyimides; and there is nothing inhis patent which would suggest how such a copolyimide could be made viathe route he discloses.

l have now discovered certain novel aromatic thermoplastic copolyimidesthat do not have the abovediscussed disadvantages of the previouslyproposed solutions to the problem of the intractability of aromaticpolyimides. They have exceptionally high thermal stability and can becoalesced at pressures which are markedly lower than those required tocoalesce the molding powders described above. And the procedure by whichthey are made is decidedly less expensive than those by which thepolymeric materials discussed above are prepared.

Another advantage of my novel copolyimides is that they can be made fromthe precursors without the use of solvents, This eliminates thediscoloration of the final product attributable to such solvents as wellas their cost.

Furthermore, as no solvents are available to catalyze the polymerizationreactions, the precursors of my novel copolyimide have very long shelflives.

Also, the absence of solvents 'makes my process much more versatile inthat! can produce molded and other articles with thick cross-sections inaddition to the films and coatings to which the processes which employsolvents are limited.

In a preferred method my novel aromatic copolyi-- mides are preparedfrom monomeric resinoids which are solid state solutions of an alkylester of 3,3,4,4'- benzophenonetetracarboxylic acid and two or morearomatic diamines which are free of aliphatic moieties. At least one ofthe diamines must be metasubstituted, and any diamines which are notmeta-substituted must be para-substituted. Also, the imide-formingfunctionalities (the amino and carboxylic moieties) should be present insubstantially equimolar amounts.

The monomeric precursors are prepared by first reacting3,3',4,4'-benzophenonetetracarboxylic acid dianhydride and an esterfyingagent to form an alkyl diester. The preferred esterfying agents aremethyl, ethyl, propyl, and isopropyl alcohols. Other alkyl alcohols canalso be used as the esterfying agent-solvent. Changing the alkyl groupof the esterfying agent effects the curing rate of the product andproperties associated with the resinous nature of the material such astackiness, drying time, etc.) Ethanol is in many cases preferred becauseof its widespread availability, low cost, lack of toxicity and otherattributes.

The esterification reaction is followed by the addition of the aromaticdiamines, which are allowed to dissolve in the reaction mixture, thetemperature being kept below the reflux temperature of the esterfyingagent and in any event below 100C to avoid polymerization.

Any excess solvent remaining after the dissolution of the diamines isremoved, leaving an amorphous unreacted resinoid which has highsolubility in low boiling solvents and also has a long shelf lifebecause of the unreacted character of its constituents. Long shelf lifeis an important attribute not possessed by polyimide precursors such asthose disclosed in the above-cited Endrey patent, for example, which areof polymeric character.

My novel precursors also have the advantage that they can be preparedsimply by mixing at controlled temperatures. Thus, they are more easilyand inexpensively prepared than the typical polyimide precursorsheretofore available.

That the precursors of the present invention are made from constituentswhich include a reactive solvent further distinguishes them frompolyimide precursors such as those disclosed in Endrey. If one attemptedto make a polyimide as disclosed herein using the Endrey teachings, thereactive solvent I necessarily employ would completely destroy andnegate the effect of the acidic catalyst which Endrey says is essentialin his process.

To some extent the novel precursors disclosed herein are similar tothose described in U.S. Pat. No. 3,506,583 issued Apr. 14, 1970, toWilliam R. Boram and myself for MONOMERIC SOLID STATE SOLU- TIONS OFCERTAIN AROMATIC DIAMINES IN DERIVATIVES OF BENZOPHENONETETRACAR-BOXYLIC ACID. However, they have a number of advantages over theprecursors disclosed in the Boram patent.

Perhaps the most important is that the precursors disclosed herein canbe converted to thermomoldable copolyimides while the homopolyimidesobtained from the precursors disclosed in the Boram patent cannot besuccessfully molded, even at high temperatures and pressures, but can beshaped only by machining and similar techniques.

The solid state solutions or resinoids of the present invention can beconverted to the corresponding copolyimides by heating them first to atemperature in the range of about 125 to about 150C., which produces afoam. This foam is ground and the resulting particulate material heatedto a temperature in the range of 300 to 325C. until it is fully cured.The cured polymer can then be placed in a mold and caused to flow atabout 330C. under moderate pressures (approximately 800 psi minimum).This temperature is well below the degradation temperature of thepolymer.

One advantage of the novel method of producing a polyimide justdescribed is that the use of costly solvents is avoided. Anotheradvantage is that the polymerization reaction requires only a simpleheating cycle and can be carried out in air.

Another advantage of the present invention is that the polymerizationprocess is self catalyzing because the condensation by-products aredriven 0E, and no reaction takes place which is competitive with theformation of the polyimide resin. Consequently, no catalysts are neededto produce the copolyimides of this invention. Furthermore, because ofthe self-catalysis, uniquely high yields of the polyimide (consistentlyabove percent) are obtained.

Another advantage of the present invention is that no chemical reactionsoccur in the molding step. Accordingly, there is no evolution ofsolvents or reaction products in this step. This permits the fabricationof dense, void-free structures. In the prior art processes such as thatdisclosed in Endrey, in contrast, the evolving solvents and/orcondensation products are trapped in the polymer, producing voids andweakening the material.

Yet another advantage of the present invention is unique hydrolyticstability at elevated temperatures. It is well-known that polyimidesabsorb water and deteriorate when exposed to high humidity or immersedin boiling water for extended periods of time. Yet it has been foundthat the copolyimides disclosed herein can even be boiled in waterforseveral hours and then placed in an oven preheated to as high as600F. without the formation of blisters or other defects and withoutshrinkage.

The unique properties just described are important in applications suchas turbomachinery pressure seals where the mating materials aresubjected to drastic and spontaneous changes of humidity and temperaturecapable of causing deformation and catastrophic failure if the materialsare not properly selected.

A further advantage of my invention is that the preparation of apolymeric precursor as is required in most previously known applicationsof polyimides is eliminated. This is because the polyimide can be formeddirectly from the monomeric resinoid.

In my method the material is for the most part converted directly fromthe monomer to the copolyimide without going through the copolyamic acidintermediate, and such intermediate as may be formed is not isolated.For applications such as molding, this method of preparing thecopolyimide is preferable for the reasons discussed previously.

However, in other applications such as surface coating, the use of apolymeric precursor may be advantageous. In such circumstances thediamines and acid derivative are heated in a solvent such as N,N'-dimethylformamide to a temperature of about C until polymerizationoccurs. The polymeric intermediate may then be isolated viaprecipitation in water followed by filtration.

In conjunction with the foregoing, in another method of preparing thenovel copolyimides described herein, the solution of diamines andtetracarboxylic acid derivative is heated in a solvent such asN,N'-dimethylformamide to reflux temperature until polymerizationoccurs. The polymer is then separated from the solvent in the mannerdescribed in the preceding paragraph. In these circumstances theisolated polymer is the copolyimide.

The therrnoplasticity of the novel copolyimides described above isattributable to randomness in the distribution of the mesomers(repeating units) in the polymer chain. More specifically, in ahomopolyimide, there is regularity of the mesomers since homopolyimideshave the structure:

where x is the number of times the unit is repeated. Such polymers aretherefore inflexible and crystalline, as a result of which theirtransition temperatures are so high that extensive oxidation occurs inmolding them. Accordingly, such polymers cannot be thermoformed.

In contrast, by using more than one diamine component, I have introducedan element of randomness into the polymer chain. This markedly reducesthe degree of crystallinity in my novel copolyimides in comparison tothat observed in the heretofore known aromatic homopolymers.

More particularly, copolymers have as many different repeating units asthere are combinations of mesomer-forming reactants. Copolymers arerepresented as follows:

- [MesomerA] [MesomerBP ,etc.

If the reaction of the components is random, x and y are different; andthe chain is characterized by a random distribution of the mesomersinvolved. As suggested above, this results in a lower degree ofcrystallinity. The reduction in crystallinity lowers the transitiontemperature of the polymer below its degradation temperature, permittingit to be thermoformed by the same techniques as used with otherthermoplastic materials.

At the same time there is no loss of thermal stability as in the priorart systems discussed above since, in contrast to such systems, there isno introduction of aliphatic moieties into the chain (polymerscontaining aliphatic moieties are thermally and oxidatively sensitive).

As will become apparent hereinafter, as few as two diamines may beemployed in the novel compolyimides of the present invention. However,it is for some applications preferred that three or more diamines beused and that at least two of these be meta-substituted. Also, if arigid polymer is desired, at least one para-substituted diamine isemployed.

That specific diamines must be employed to make polyimides of thecharacter disclosed herein could in no way have been predicted from theprior art. Typical polyimide patents such as Endrey list a number ofdiamines which can be used in the polymers they disclose and then statethat mixtures of the listed diamines can also be employed although theydo not disclose how or which mixtures. Thus, these prior art patentsmerely invite an arbitrary selection of the listed diamines which wouldonly by pure chance fulfill the criteria set forth above.

Copolyim ides made from 3,3 ',4,4- benzophenonetetracarboxylic acid,dialkyl esters; 2,6- diaminopyridine; 3,3'-diaminodiphenylsulfone; and4,4'-diaminodiphenylsulfone with the amine functionalities, theimide-forming dicarboxylic acid derivative, and the diester formingmoieties present in substantially equimolar amounts are preferred formany applications. These particular polyimides form stable monomericprecursors which have long shelf lives, and

this system lends itself to a very economical process of preparation.Also, copolyimides prepared from the foregoing mixtures can bethermomolded into very complicated shapes. They also have lowcoefficients of friction and can be readily shaped into articles such asrollers, inserts for rods, and actuators where extremely high surfaceflatness and smoothness are required.

However, for other applications, systems containing different and both asmaller and greater number of diamines may instead be employed.

In any event, it is preferred (though not essential) that the diaminesemployed be selected from those which have primary amine basedisassociation constants lower than 10 (all of those just identifiedhave primary amine base disassociation constants on the order of 10'").This is important from an economic point-of-view.

In those systems employing more reactive diamines the amino groups ofthe monomers tend to react rapidly with atmospheric oxygen and thecarboxylic acid groups of the monomers, even at room temperature. Thus,their shelf life tends to be extremely limited.

In constrast, the preferred precursors of the present invention are notappreciably subject to oxidative degradation, even at the elevatedtemperatures which exist during the curing cycle. Accordingly, they havelong shelf lives. Also, they can be polymerized simply by heating themin air, which is a comparatively inexpensive step to perform.

Notwithstanding the foregoing, more reactive diamines can be employed inthe monomeric precursors of the present invention where otherconsiderations outweigh the advantages of long shelf life andprocessability in air. Illustrative of the aromatic metaandpara-substituted diamines which may be employed instead of or incombination with one or more of the less reactive diamines referred toabove in such circumstances are:

3 ,3 '-Diaminodiphenyl ether 4,4-Diaminodiphenyl etherm-Phenylenediamine 4,4-Bisoxyaniline p-Phenylenediamine The resinoids ofthis invention are useful in the preparation of foam structures inaddition to the dense, void-free structures described above. To form afoam, the monomeric resinoid precursor is heated in the absence ofsolvents and typically in a circulating air oven from room temperatureto on the order of 300 to 325C. The materials obtained are low densitycopolyimide foams, which are highly resilient and have uniform cellsize. Thesefoams are useful as acoustical, thermal, and electricalinsulators at high temperatures. In addition, they are useful in radarstructures because of their transparency characteristics at radarfrequencies.

The resinoids described herein are also useful in other applications.For example, they can be used in solution for impregnation purposes andin the absence of solvents for the fabrication of composites.

From the foregoing it will be apparent that one important and primaryobject of the present invention resides in the provision of novelcopolyimides which have the advantages of known aromatic homopolyimidessuch as chemical inertness, high strength, re-

sistance to high temperatures, etc. and which, in addition, arethermoformable.

Another important and primary object of the present invention is theprovision of novel precursors which can be converted simply by heatinginto resilient copolyimide foams.

Yet other important, primary objects of this invention reside in theprovision of novel monomeric, resinoidlike copolyimide precursors and inthe provision of novel methods for preparingsuch precursors and forconverting the precursors into thermoformable copolyimides, intoresilient copolyimide foams, and into copolyamic acid precursors of thecopolyimides.

Other objects and advantages and additional novel features of thepresent invention will be apparent to those skilled in the relevant artsfrom the foregoing general description of the invention, from theappended claims, and from the following examples, which are intended toillustrate and not restrict the scope of the invention.

EXAMPLE I 3,3',4,4'-Benzophenonetetracarboxylic acid dianhydride (32.22g, 0.10 M) was dissolved in125 mls. of ethanol to convert it to thediester. 2,6- Pyridinediamine (2,6-diaminopyridine) (4.37 g, 0.04 M),3,3-diaminodiphenylsulfone (9.93 g, 0.04 M), and4,4-diaminodiphenylsulfone (4.97 g, 0.02 M) and 125 mls. of ethanol wereadded to the diester solution at room temperature. The mixture wasstirred and heated until all solids were dissolved (30 minutes atapproximately 30C). The solution was filtered by gravity, and thesolvent was removed in a rotary evaporator and later in a vacuum oven at78C. The resulting foam was powdered and vacuum dried for 1 hour at80C., producing a monomeric resinoid powder.

The monomeric resinoid thus produced was found to have a long shelflife. It remained a free flowingpowder after storage for 4 months. Also,it remained highly soluble in ethanol and acetone.

EXAMPLE [I The resinoid of Example I was heated in a circulating airoven from room temperature to 150C. in minutes. A foam structure wasformed. This was homogeneous, had uniform cell size, and was brightyellow. The foam was powdered and heated in air at 315C. for 10 hours.

The resulting powder was finely ground, placed in a steel die, andheated to 330C. under 900 psi. After 30 minutes, the sample was cooledand removed from the die. A transparent reddish pellet of material wasobtained.

The material EXAMPLE III a Rockwell Hardness (B Scale) of 73 and adensity of 1.4 g/cm. This was a surprising result since most polyimideshave a Rockwell B Hardness of about 5.

EXAMPLE III A monomeric resinoid was prepared by the procedure describedin Example I but using only two diamines. Specifically, 3,3,4,4'-benzophenonetetracarboxylic acid dianhydride (32.22 g, 0.10 M) wasdissolved in 150 mls of ethanol to convert it to the diester.2,6-Pyridinediamine (5.46 g, 0.05 M) and 4,4'-diaminodiphenyl sulfone(12.42 g, 0.05 M) were added along with 100 mls of ethanol The mixturewas stirred and heated until homogeneous (at about 40C.) and thenfiltered by gravity. The solvent was removed under reduced pressure,resulting in a resinoid similar to that obtained in Example 1.

This resinoid was placed on a ceramic brick in an oven in air at 315C.After 15 minutes, a foam structure was obtained. This structure wasresilient, of low density (approximately 0.03 gm/cm and light yellow incolor. This material had an open cell structure and was found useful asan acoustical insulation.

EXAMPLE IV The resinoid of Example III was powdered and mixed withaluminum powder in a 4:1 ratio by weight. The mixture was heated to315C. This produced a foam structure in which the aluminum washomogeneously dispersed.

Lead, copper, asbestos, silica, boron and tin powders were used asfillers with similar results. Other materials may be added as requiredto produce combinations with specific properties and applications.

EXAMPLE V The resinoid of Example I was cured as described in ExampleII. The cured copolyimide powder was mixed with boron fibers, placed ina mold, and pressed at 330C. under 850 to 1000 psi. The material formeda pellet which was transparent and in which the boron fibers could beseen.

EXAMPLE Vl 2,6-Diaminopyridine (4.37 g, 0.04 M), 3,3-diaminodiphenylsulfone, (9.93 g, 0.04 M), and 4,4-diaminodiphenylsulfone (4.96 g, 0.02 M) were added to 200 g ofpolyphosphoric acid in a round bottom flask. The mixture was stirred andheated to 60C.; and,

when it had become homogeneous, it was cooled 'to 30C.3,3,4,4'-Benzophenonetetracarboxylic acid dianhydride (32.22 g, 0.10 M)was then added, and the mixture was heated at 230C. until it becamehomogeneous.

The reaction mixture was cooled to approximately C. and added to 3liters of distilled water. The resulting solids were isolated byfiltration, washed three times with distilled water, and then washedthree times with reagent ethanol. The polymer thus obtained was driedand heated to 315C. for 5 hours.

A portion of this polymer was placed in a mold and subjected to 800 psiat 330C. for 10 minutes. The polymer was found to flow under theseconditions.

EXAMPLE VII The following diamines were added to 200 gms ofpolyphosphoric acid as in Example VI.

m-Phenylenediamine 3.24 g 0.03 M 2,6-Diaminopyridine 3.27 g 0.03 M4,4'-Diaminodiphenylsulfone 4.97 g 0.02 M 4,4'-Bisoxyaniline 4.00 g 0.02M

The reaction mixture was stirred at 60C. and then cooled to 30C.Benzophenonetetracarboxylic acid dianhydride (32.22 g, 0.10 M) was thenadded and the mixture heated to 260C. This produced a polymer which wasisolated and washed as in Example VI and found to flow undertemperature-pressure conditions similar to those described in ExampleVI.

EXAMPLE VIII The following diamines were dissolved in 250 mls ofN,N'-dimethylforrnamide in an inert atmosphere:

m-Phenylenediamine 4.32 g 0.04 M 2,6 Diaminopyridine 4.37 g 0.04 M4,4'-Bisoxyaniline 4.00 g 0.02 M

When the mixture became homogeneous it was cooled to 15C.3,3',4,4'-Benzo'phenonetetracarboxylic acid dianhydride (32.22 g, 0.10M) was added and stirred until it dissolved as the mixture was allowedto come to room temperature. The mixture was then heated to refluxtemperature, cooled, and poured into 2 liters of water. This formed apolymer which was isolated by vacuum filtration and washed repeatedlywith water and ethanol. The polymer was then dried and heated to 315C.in argon.

A sample of this polymer was pressed at 330C. and approximately 1,000psi and found to flow in the same manner as the material described inExample II.

In the foregoing example the polymer isolated from theN,N'-dimethylformamide solvent was a mixture of the copolyimide and thecorresponding copolyarnic acid intermediate. Had it been desired torecover the material in the form of the intermediate rather than as amixture of the intermediate and copolyimide, the diamine-dianhydridemixture would have been heated to a lower temperature (on the order of100C.) rather than to reflux temperature.

As discussed previously, copolyimides employing three polyimides arepreferred for some applications of the present invention. However, ifdesired, two, or more than three, diamines may also be used wherecircumstances warrant as shown by the examples which follow.

EXAMPLE IX 3,3',4,4-Benzophenonetetracarboxylic acid dianhydride (32.23g, 0.10 M) was added to 150 mls of reagent ethanol. The mixture washeated and refluxed until the solids were dissolved. 2,6-Diaminopyridine(5.45 g, 0.05 M) and 3,3'-diaminodiphenylsulfone (24.81 g, 0.05 M) wereadded to the mixture at room temperature along with 100 ml of ethanolThe mixture was stirred and heated to about 40C. until the solidsdissolved. The solution was then filtered and the solvent removed underreduced pressure at temperatures up to 71C. The resulting material waspowdered and dried overnight. Then it was polymerized in an argonatmosphere by heating to 150C. The polymeric material was ground andheated to 308C, producing a copolyimide which was causedto flow in amold at 328C. and 900 psi. The molded part was transparent and quiteflexible and had a density of 1.4 g/cm EXAMPLE X3,3',4,4'-Benzophenonetetracarboxylic acid dianhydride (32.23 g, 0.10 M)was dissolved in ethanol as in theprevious example. 4,4-Bisoxyaniline (2g, 0.01 M), 2,6-diaminopyridine (4.37 g, 0.04 M), 4,4-diaminodiphenylsulfone (9.92 g, 0.04 M), and 3,3- diaminodiphenylsulfone(2.48 g, 0.01 M) were dissolved in the diester solution as in theprevious example. The solution was filtered, and the solvent was removedunder reduced pressure. The precursor thus recovered was heated toeffect polymerization in an inert, argon atmosphere; and the resultingcopolymer was caused to flow in a mold at 328C. and 900 psi. The moldeddisc obtained was somewhat darker than but otherwise similar to thatobtained in the previous example.

The foregoing example also demonstrates that diamines having a primaryamine base disassociation constant exceeding 10' may be employed in thepractice of the present invention if polymerization is effected in aninert atmosphere.

The materials produced by the procedures described in Examples II-Xabove were determined in each case to be a copolyimide by infraredspectrometry. In each instance imide bonds where present in the spectraand amide bonds were absent. Also, in each case, the

material was caused to fiow without evidence of degradation.

EXAMPLE XI To demonstrate the considerable differences between a typicalhomopolyimide prepared from a precursor of the type disclosed in theBoram patent and a similar copolyimide made from a precursor of the typedisclosed herein, test specimens 5 inches long, one-half inch wide, andapproximately one-eighth inch or more thick were molded from both typesof polyimides and subjected to various tests of physical properties.

One set of test specimens was made from a homopolyimide prepared from 3,3 ,4,4 benzophenonetetracarboxylic dianhydride, ethyl alcohol, and4,4'-diaminodiphenylsulfone following the procedure described in ExampleIII of U.S. Pat. No. 3,506,583 and the curing cycle described in column10.

A second set of samples was prepared from a copolyimide made from 3 ,3',4,4 benzophenonetetracarboxylic dianhydride, ethyl alcohol,4,4-diaminodiphenylsulfone, and 3,3 diaminodiphenyl sulfone followingthe procedures set forth in Examples I and II herein.

The following results were observed:

Boram Polyimide Polyimide of Present Invention Molding Pressure Requiredfor coalescence (psi) 10,000-20,000 GOO-5,000 Molding TemperatureRequired for coalescence ("F.) 700-800 500-700 Moldability Very poorExcellent Flexural Strength (psi) LOGO-2,000 15,000-

' 25 ,000 Flexural Modulus 1-2 X 10 6-9 X l0 Tensile Strength (psi)5,000l0,000 20,000-

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription; and all changes which come within the meaning ad range ofequivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by Letters Patent is:

1. A copolyimide which is a condensation product of an alkyl ester of3,3,4,4'-benzophenonetetracarboxylic acid and at least twoaromatic'diamines which are free of aliphatic moieties, at least one ofsaid diamines being meta-substituted and all of the diamines beingeither metaor para-substituted.

2. A resinoidlike, copolyimide precursor which is essentially anunreacted mixture of an alkyl diester of3,3,4,4-benzophenonetetracarboxylic acid and at least two aromaticdiamines which are free of aliphatic moieties, at least one of saiddiamines being meta-substituted, any diamines which are notmetasubstitutedbeing para-substituted, and the ratio of the imideforming functionalities being substantially equimolar.

3. A precursor according to claim 2, wherein the diamines are selectedfrom those having primary amine base disassociation constants notexceeding l 4. A precursor according to claim 2, wherein the alkyldiester is selected from methyl, ethyl, propyl, and isopropyl diestersof 3,3,4,4'-benzophenonetetracarboxylic acid.

5. A precursor according to claim 2, wherein at least one of thediamines is para-substituted.

'6. A precursor according to claim 2, wherein the diamines are or areselected from or include 2,6- diaminopyridine,3,3'-diaminodiphenylsulfone; and 4,4-diaminodiphenylsulfone.

7. A precursor according to claim 2, which contains two diamines.

8. A precursor according to claim 2, which contains three diamines.

9. A precursor according to claim 2, which contains more than threediamines.

10. A precursor according to claim 2, in which the alkyl diester is theethyl diester of 3,3',4.4- benzophenonetetracarboxylic acid.

11. A precursor according to claim 2, containing at least onediaminewith a primary amine base disassociation constant which exceeds10* 12. The process of preparing a monomeric, resinoidlike precursorwhich is an unreacted mixture of components capable of being polymerizedto a thermoplastic copolyimide, comprising the steps of dissolving3,3',4,4-benzophe'nonetetracarboxylic acid dianhydride in an esterfyingagent to convert the dianhydride to an alkyl ester of 3,3,4,4-benzophenonetetracarboxylic acid; dissolving in the product thus formedat least two diamines which are free of aliphatic moieties in amountssuch that the ratio of the imide forming functionalities issubstantially equimolar, at least one of the diamines beingmeta-substituted and any diamines which are not meta-substituted beingpara-substituted; and removing from the mixture any excess esterfyingagent, the temperature of the mixture during the dissolution of thediamines and the removal of the solvent being kept below 100C. to avoidcondensation reactions between the diester and the diamines.

. The process of claim 12, wherein the esterfying

2. A resinoidlike, copolyimide precursor which is essentially anunreacted mixture of an alkyl diester of3,3'',4,4''-benzophenonetetracarboxylic acid and at least two aromaticdiamines which are free of aliphatic moieties, at least one of saiddiamines being meta-substituted, any diamines which are notmeta-substituted being para-substituted, and the ratio of the imideforming functionalities being substantially equimolar.
 3. A precursoraccording to claim 2, wherein the diamines are selected from thosehaving primary amine base disassociation constants not exceeding 10 10.4. A precursor according to claim 2, wherein the alkyl diester isselected from methyl, ethyl, propyl, and isopropyl diesters of3,3'',4,4''-benzophenonetetracarboxylic acid.
 5. A precursor accordingto claim 2, wherein at least one of the diamines is para-substituted. 6.A precursor according to claim 2, wherein the diamines are or areselected from or include 2,6-diaminopyridine,3,3''-diaminodiphenylsulfone; and 4,4''-diaminodiphenylsulfone.
 7. Aprecursor according to claim 2, which contains two diamines.
 8. Aprecursor according to claim 2, which contains three diamines.
 9. Aprecursor according to claim 2, which contains more than three diamines.10. A precursor according to claim 2, in which the alkyl diester is theethyl diester of 3,3'',4,4''-benzophenonetetracarboxylic acid.
 11. Aprecursor according to claim 2, containing at least one diamine with aprimary amine base disassociation constant which exceeds 10
 10. 12. Theprocess of preparing a monomeric, resinoidlike precursor which is anunreactEd mixture of components capable of being polymerized to athermoplastic copolyimide, comprising the steps of dissolving3,3'',4,4''-benzophenonetetracarboxylic acid dianhydride in anesterfying agent to convert the dianhydride to an alkyl ester of3,3'',4,4''-benzophenonetetracarboxylic acid; dissolving in the productthus formed at least two diamines which are free of aliphatic moietiesin amounts such that the ratio of the imide forming functionalities issubstantially equimolar, at least one of the diamines beingmeta-substituted and any diamines which are not meta-substituted beingpara-substituted; and removing from the mixture any excess esterfyingagent, the temperature of the mixture during the dissolution of thediamines and the removal of the solvent being kept below 100*C. to avoidcondensation reactions between the diester and the diamines.
 13. Theprocess of claim 12, wherein the esterfying agent is methyl, ethyl,propyl, or isopropyl alcohol.
 14. The process of claim 12, wherein theesterfying agent is ethyl alcohol.
 15. The process of claim 12, whereinthe diamines all have primary amine base disassociation constants whichdo not exceed 10
 10. 16. The method of claim 12, wherein the diaminesare or are selected from or include 2,6-diaminopyridine,3,3''-diaminodiphenylsulfone; and 4,4''-diaminodiphenylsulfone.